In my aforementioned patent applications, I described a soft airfoil wind turbine comprising a large wheel carrying a plurality of soft airfoils disposed between the wheel rim and hub to capture the wind. The wheel is suspended by cables from its rim, which rim rides on rollers of a carriage assembly. The carriage assembly, in turn, is movable around a circular track, so that the wheel's orientation relative to the wind may be changed.
The wheel has a plurality of uniformly spaced, radial, rotary spindles, about which are furled the soft airfoils. Structural support for the spindle assembly is provided in substantial part by a plurality of angularly spaced diametral brace cables, or elongate support members, extending between the wheel hub and rim. Associated with each elongate support member is a boom member and an elongate rotary spindle. The boom member has one free end for supporting a portion of the soft airfoil spaced from the spindle and has its other end pivotally mounted to the associated elongate support member. The angle of the airfoil relative to the plane of the wheel is altered by pivoting the boom member. The associated spindle is located alongside the elongate support member. In one form, the spindle is fixedly mounted at its opposite ends to the hub and rim. In another form, the end of the spindle adjacent the wheel rim is mounted for pivotal movement about the elongate support member.
While this supporting structure for the spindle is very satisfactory in many applications, in a very large wind turbine application in which the elongate spindle reaches a length of 150 or more feet, for instance, the lateral loading of the spindle by wind pressure against the airfoil carried thereby tends to cause the spindle to be flexed, or bow out, along its length. This results in a loss of power. Further, if the deflection is too great, the spindle may break or the smooth furling and unfurling by rotation of the spindle is interfered with.
In sailboats, this problem is not so severe because the spindle lengths are not so great as in a large wind turbine. Consequently, the problem of bowing has been addressed by placing the spindle, usually a cable or rod, under high tensile loading. This generally requires that the spindle be supported by a compressive member, such as a mast. Further, this tensile loading makes rotation of the spindle more difficult and increases mechanical wear.
Another approach used in both sailboats and wind turbines has been to make the airfoil carrying member more massive, and thus inherently more capable of resisting the lateral deflection forces. Such an approach, however, undesirably adds weight and cost to the structure. Examples of the use of massive structures such as this in wind turbine applications are shown in U.S. Pat. Nos. 704,506 issued July 15, 1902 to Bruneau and 242,211 of McIlvaine. In German Pat. No. 2,642,570 issued to Schnitzer in March, 1978, excessive length of the spindle is avoided by making it a chordal rather than a radial member.
Sailboat structures are also shown in which a rotatable, sail carrying spindle is mounted within a hollow mast or mast sleeve having an elongate slot for passage of the sail. Again, these masts are relatively massive, compressive support members. Accordingly, they are impractical for use in a large wind turbine application. Examples of these hollow masts, or sleeve structures, are shown in U.S. Pat. Nos. 4,149,482 issued to Hoyt on Apr. 17, 1979; 4,061,101 issued to Cook, the present inventor, on Dec. 6, 1977; 3,835,804 issued to Jackson on Sept. 17, 1974; 4,116,152 issued to Larsson on Sept. 26, 1978; and 4,030,436 issued to Hood et al. on June 21, 1977.
In such sailboat structures, one end of the boom is generally mounted beneath the sail to the mast for pivotal movement about an axis coincident with that of the spindle assembly. This is done so that the distance between the sail clew supported at the free end of the boom and the spindle assembly remains substantially constant as the boom is rotated to avoid stretching of the sail. In fact the sail pivots about an axis along the periphery of the furled portion of the sail or along the line of emergence of the sail from the hollow mast which are spaced from the spindle axis. In addition, as the boom pivots in the rotational direction that the sail is furled about the spindle, the unfurled portion of the sail wraps around the furled portion of the sail and spindle and is shortened. When rotated in the opposite direction, the unfurled portion is lengthened.
The known technique of avoiding straining or luffing of the sail under the circumstances has been to move the location of the clew along the boom relative to the spindle as the boom rotates. Typically, this has been done by manually adjusting the outhaul line before or during the pivoting of the boom. Alternately, in my aforementioned U.S. Pat. No. 4,061,101, I provide a furling system in which the outhaul line and thus the relative location of the clew along the boom are automatically adjusted when the boom rotates.
In a wind turbine application which requires rapid simultaneous adjustment of the angle to the wind of a plurality of airfoils in a spinning wheel, avoidance of the problem of stretching the sail is more critical. Since the massive supporting structures of sailboats are to be avoided in a wind turbine application, the solution to this problem is more difficult and other approaches are required. In my earliest aforementioned patent application, the problem of supporting the clew for pivotal movement about the spindle axis in a wind turbine application is achieved through use of an arcuate track to which the clew is movably mounted instead of a boom.
In my later aforementioned application, this problem is solved in part by mounting the end of the spindle which is adjacent the rim for pivotal movement about the support member to which a boom member assembly is also pivotally mounted. Since a deck is not available to secure the free end of the boom against movement parallel to the spindle, this boom member assembly has a pair of boom members connected at their free ends to form a "V" and mounted at their other ends for pivotal movement about the support member. While one of these boom members is located adjacent one end of the spindle assembly, the other boom member must be mounted intermediate the ends of the spindle assembly.
While these prior structures of mine function satisfactorily in large wind turbine applications I have found that means for providing lateral support or constraint for the elongate spindle must also be incorporated into the spindle assembly design. In addition to providing the needed lateral support, such constraint means must not interfere with the smooth furling and unfurling of the airfoil or the rapid pivotal movements of the boom and unfurled airfoil carried thereby.